Method for protection of stone with substantially amorphous fluoropolymers

ABSTRACT

This invention concerns the application of substantially amorphous fluoropolymer compositions to stone so as to protect the stone from the deleterious effects of water and pollution.

This application claims the benefit of Provisional Application No.60/204,447, filed May 15, 2000.

FIELD OF THE INVENTION

This invention concerns the application of substantially amorphousfluoropolymer compositions to stone in order to protect the stone fromthe deleterious effects of water and pollution. This invention is usefulfor the preservation of historic monuments, buildings, and sculpturesand for the construction of weather and pollution resistant stoneconstruction materials.

TECHNICAL BACKGROUND OF THE INVENTION

It has long been recognized that a combination of man-made and naturalatmospheric conditions are having deleterious effects on stonestructures including many monuments of considerable artistic andhistorical importance. A variety of efforts have been undertaken overthe years to identify ways to protect and preserve these structures, butthese efforts have met with only partial success. Most recently, awell-organized and concerted effort has been undertaken by Piacenti andcoworkers, with sponsorship from the Target Project for the CulturalHeritage of the Consiglio Nazionale della Recerche of Rome, Italy, andrepresents the current state of the art.

The deleterious effects of water, in the form of both rainfall andcondensation, are of primary concern, although organic pollutants arealso of concern. For example, all building materials are subject tostress and concomitant cracking resulting from penetration by waterfollowed by cycles of freezing and thawing. Also, water in combinationwith CO₂, which occurs naturally, and nitrous and sulfurous gases, whichare man-made pollutants, forms acids which rapidly eat away at thestone.

A successful attack on the problem will necessitate some tradeoffs.While it is highly desirable to minimize the contact between water andstone, by achieving maximum water repellency, it is also necessary toprovide high water vapor permeability in order to permit venting of thatwater which finds its way into the microstructure of the stone.Substances with high permeability to water vapor are often not those ofthe highest water repellency. High resistance to acid and abrasion arealso of considerable importance. Furthermore, cost of materials is afactor in any practical application. And, for several reasons, thesmaller the amount of material required to achieve the desired effect,the better.

There are other tradeoffs. For example, it is particularly desirablethat the coating material coat but preferably not block the pores. Toachieve this, a coating viscosity should be in a range which permitswetting of the pores via capillary action. High wetting is also requiredto ensure thorough and uniform coverage. However, the coating must beprovided with sufficient adhesion to the outside surface upon which itis deposited that at least some amount will remain thereon.

Other requirements for such materials include chemical inertness, lowvolatility, photooxidative stability and thermal stability. There shouldalso be sufficient solubility in environmentally friendly solvents forthe purposes of both application and subsequent removal if warranted.The coating must also be clear and colorless, and remain so for itslifetime. In the current state of the art, the application solvent ofchoice is supercritical CO₂, as described in Carbonell et al., WO99/19080.

In a series of patents, U.S. Pat. No. 4,499,146, U.S. Pat. No.4,746,550, U.S. Pat. No. 4,745,009, U.S. Pat. No. 4,902,538, Piacenti etal. disclose compositions based upon perfluoropolyethers havingmolecular weights in the range of 500-5000 for use in the protection ofstone from the effects of water and atmospheric pollutants. In the artof Piacenti, excellent combinations of water repellency and water vaporpermeability are achieved.

In U.S. Pat. No. 4,902,538, good results are achieved in compositionshaving highly crystalline particles of polytetrafluoroethylene andcopolymers thereof intermixed with the perfluoropolyethers. However,when stone of porosity of greater than ca. 30% is treated, impracticallyhigh levels of coating material are required to achieve the desiredcoverage with the desired water repellency. Levels in the range of atleast 150 g/m² are disclosed, more than 10 times the amount required forlow-porosity marble. The effect of this high coating level onpermeability is not disclosed. Its effect on cost, however, is clearlyundesirable. Furthermore, use of highly crystalline polymers, such aspolytetrafluoroethylene, is undesirable because, unless they aresintered at high temperatures, they will be too readily susceptible toremoval from the treated surface by abrasion and erosion. Further still,they are not readily soluble in the delivery medium of choice, CO₂, orany other desirable medium.

Also disclosed in the art in Piacenti et al., U.S. Pat. No. 4,764,431,are copolymers of vinylidene fluoride which are less effective than theperfluoropolyethers.

Fluorinated acrylic polymers are disclosed by Ciardelli et al., Prog. inOrg. Coatings, 32, 43-50 (1997). The polymers disclosed therein arecharacterized by hydrocarbon backbones and fluorinated pendant groups.These polymers exhibit similar functionality to the perfluoropolyethers.

Guidetti et al. disclose the use of polyfluorosubstantially amorphousfluoropolymers for protecting stone in “Polyfluorosubstantiallyamorphous fluoropolymers as stone protectives”, 7th InternationalCongress on Deterioration and Conservation of Stone, 1053-62 (1992).

There is considerable incentive in the art to discover new materialswhich possess several of the above attributes desired for theapplication.

SUMMARY OF THE INVENTION

The present invention provides a process for protecting stonecomprising:

contacting stone with a substantially amorphous fluoropolymer comprising

(a) at least about 10 mole percent of repeat units of the formula

—CF₂—CF(CF₃)—  (I);

(b) 0 to 50 mole percent of repeat units of the formula

—CF₂—CF₂—  (II);

 and

(c) about 10 to about 75 mol-% of repeat units selected from the groupconsisting of:

—CF₂CFX—, —CH₂—CHR—, —CH₂CF₂—  (III)

and mixtures thereof wherein X is H or perfluoroalkoxy having 1-20carbons, and R is H, alkyl, alkoxy, perfluoroalkyl, F. Furthermore, thebackbone H:F ratio should be about 0.3 to about 1.0. If the polymer MWis less than about 5000, the H:F ratio can be broader.

DETAILED DESCRIPTION

For the purpose of the present invention, the term “stone” means anatural stone used in construction or sculpture (such as granite,marble, limestone, or sandstone) as well as tile, cement, brick, stucco,and the like.

The method of the present invention provides surprising benefits overthe methods of the art. In the method of the present invention, asubstantially amorphous fluoropolymer composition is employed as acoating agent for stone in order to provide high liquid moisturebarrier, good moisture vapor permeability, and resistance toenvironmental pollutants. The non-fugitive, very low areal densitycoating formed on the stone surface is surprisingly effective over thematerials of the art. Furthermore, the substantially amorphousfluoropolymer of the present invention is readily soluble in a varietyof solvents by virtue of its amorphous nature, and is thereby bothreadily applied in the form of an environmentally friendly solution andreadily removed by conventional solvents should that be deemed necessaryafter application. Further still, the highly desirable effects of themethod of the present invention are achieved employing a substantiallyamorphous fluoropolymer in relatively small quantities in order toachieve the desired combination of water vapor permeability and liquidwater resistance.

The polymers suitable for use in the present invention are substantiallyamorphous, in contrast to most fluorinated polymers in common use whichare known to be moderately to highly crystalline. One of skill in theart will appreciate that the degree of polymer crystallinity which canbe tolerated in a given situation will depend upon the specific polymerstructure, solvents, other adjuvants, application methods, requisites ofthe particular application, and substrate in a given practicalembodiment of the invention. For the purpose of the present invention,amorphous polymers suitable for the practice of the invention mayexhibit a melting endotherm having an associated heat of fusion nogreater than 5 J/g, preferably no greater than 2 J/g, more preferably nogreater than 1 J/g, at a temperature above about 20° C.

More preferably the polymers employed for the practice of the inventionwill exhibit no melting endotherm above about 100° C. Most preferablythe polymers employed for the practice of the invention will exhibit nomelting endotherm whatever.

For the purpose of the present invention, the heat of fusion isdetermined by differential scanning calorimetry (DSC) at a heating rateof 10° C./min, according to ASTM D4591-97.

DSC is also the technique of choice for determining the glass transitiontemperature. Glass transition temperatures of the polymer are preferablyno higher than 30° C., most preferably no higher than 20° C. Glasstransitions should be set by the methods herein described so that thepolymer will not undergo repeated transitions while in place on thestone.

The polymers suitable for use in the present invention may be made via acontinuous polymerization process, for example according to theteachings of Anolick et al., U.S. Pat. No. 5,663,255. Continuouspolymerization reactors include continuous stirred tank reactors andpipeline (tubular) reactors, both of which are well-known in the art.The process is run at a pressure of about 41 to about 690 MPa,especially about 69 to about 103 MPa. At lower pressures the molecularweight of the polymers formed and the conversion of monomers to polymerboth tend to decrease. Solvents can be used in the reactor. so that apolymer solution may be made in a single step. When solvents are used itis preferred that they be essentially inert under process conditions.Useful solvents include perfluorodimethylcyclobutane andperfluoro(n-butyltetrahydrofuran). A particularly useful solvent is CO₂.

The polymer is soluble in the monomer(s) under the process conditions.Therefore, one method of polymer isolation is to reduce the pressurebelow that required for solution of the polymer, and isolate the polymerfrom that, as by decantation, filtration or centrifugation.

The apparatus for running the polymerization may be any suitablepressure apparatus in which the reactant and products streams may beadded and removed at appropriate rates. Thus the apparatus may be astirred or unstirred autoclave, a pipeline type reactor, or othersuitable apparatus. Agitation is not necessary, but preferable,especially to obtain polymers with low polydispersity. The material ofconstruction should be suitable for the process ingredients, and metalssuch as stainless steel are often suitable.

The polymerization is carried out above about 200° C., and mostpreferably from about 250° to about 400° C. The initiator is chosen sothat it will generate active free radicals at the temperature at whichthe polymerization is carried out. Such free radical sources,particularly those suitable for hydrocarbon vinyl monomers at much lowertemperatures, are known to one of skill in the art, see for instance J.Brandrup, et al., Ed., Polymer Handbook, 3rd Ed., John Wiley & Sons, NewYork, 1989, p. II/1 to II/65. The preferred temperature for running theinstant process depends on both the monomers and the initiator and isoften a compromise between raising temperature to favor highproductivities and high conversions and lowering temperature to minimizechain transfer and monomer degradation. For the copolymerization of HFPwith TFE, for example, where chain transfer is not a problem, initiationby C₂F₅SO₂ C₂F₅ is a good choice on account of the very highproductivities it affords at 400° C. For the polymerization ofHFP/TFE/PMVE, however, where PMVE chain transfer is of prime concern,NF₃ which retains good efficiency at 250° C., is an excellent choice forinitiator. Suitable free radical initiators include NF₃, R_(f)NF₂, R_(f)²NF, R_(f) ³N, R¹N═NR¹, R_(f)OOR_(f), perfluoropiperazine, and hinderedperfluorocarbons of the formula C_(n)F_(2n+2) wherein each R_(f) isindependently perfluoroalkyl, preferably containing 1 to 20 carbonatoms, hindered perfluoroalkenes of the formula C_(n)F_(2n),perfluoro(dialkylsulfones) of the formula R¹SO₂R¹, perfluoroalkyliodides of the formula R¹I, perfluoroalkylene diiodides of the formulaIRI where the two iodides are not vicinal or geminal,perfluoro(dialkyldisulfides) R¹SSR¹, and perfluoroalkyl compoundscontaining nitrogen-sulfur bonds of the formula R¹² NSR¹, wherein eachR¹ is independently saturated perfluorohydrocarbyl optionally containingone or more ether groups, isolated iodine, bromine or chlorinesubstituents, or perfluoroamino groups. By “saturatedperfluorohydrocarbyl” is meant a univalent radical containing onlycarbon and fluorine and no unsaturated carbon—carbon bonds. The activityof any particular initiator molecule may be readily determined byminimal experimentation.

Preferred initiators are NF₃R_(f) ²NF, R_(f)NF₂, perfluoropiperazine,perfluoro(dialkylsulfones), i.e., R¹SO₂R¹, and hinderedperfluorocarbons. NF₃ is an especially preferred initiator. If highermolecular weight polymers are desired, the initiator should preferablynot have any groups present in its structure that cause any substantialchain transfer or termination during the polymerization. Such groupsusually include, for instance, organic bromides or iodides orcarbon-hydrogen bonds.

A useful range of initiator concentration has been found to be about0.003 to about 0.5 g of initiator/kg monomer, preferably about 0.1 toabout 0.3 g/kg. Higher or lower amounts are also useful depending uponthe initiator, the monomers, goal molecular weights, process equipment,and process conditions used, and can readily be determined byexperimentation. The initiator may be added to the reactor as a solutionin the monomer(s).

Various comonomers (III) may be used in the polymerization process, andbe incorporated into the polymer. Perfluoro(alkyl vinyl ethers) andperfluorinated terminal alkenes, each optionally substitited with ether,cyano, halo (other than fluorine), sulfonyl halide, hydrogen or estergroups may be used. Also unfluorinated or partially fluorinated olefinsor vinyl ethers, optionally substituted as above, may also be used.Useful comonomers include CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F, ethylene,vinylidene fluoride, CH₂═CHO(C═O)CF₃, methyl vinyl ether, CFCl═CF₂,CH₂═CFCF₃, CH₂═CHCF₃, CH₂═CHCF₂ CF₂ CF₂ CF₃, CH₂═CHCF₂ CF₂ Br, CF₂═CFCF₂CN, and CF₂═CFCF₂ OCF₂ CF₂ SO₂F. Any combination of ethylene, vinylidenefluoride, perfluoroalkylvinyl ethers, perfluorobutylethylene, alkylvinylethers and/or vinyl fluoride is preferred.

The properties of the polymer will be affected not only by the overallcomposition of the polymer, but by the distribution of the variousmonomer units in the polymer. The instant process yields a polymer inwhich the monomer units are fairly uniformly distributed in the polymer,which gives polymer with consistent properties. One measure of polymeruniformity is randomness of the monomer units in the polymer. A measureof this is relative amounts of isolated repeat units, diads, triads etc.By diads and triads are meant instances in which two or three repeatunits from the same monomer, respectively, occur in the polymer.

Many of the polymers made by the process described herein haverelatively small amounts of triads of repeat unit (I), which is ofcourse derived from HFP. Thus, in such polymers, less than 20 molepercent of (I) is in the form of triads, and preferably less than about15% and more preferably less than about 10%. As would be expected, inpolymers with higher amounts of (I), there is a tendency towards highertriad content. The amount of triads in the polymer can be determined by¹⁹F NMR.

Polymers described herein often have a polydispersity of less than 5,preferably less than 4. Repeat units (III) help suppress crystallizationand provides for a lower glass transition temperature. Preferredmonomers for repeat unit (III) are vinyl fluoride, vinylidene fluoride,perfluoroalkylvinyl ethers, perfluorobutylethylene, alkylvinyl ethersand/or ethylene, with a mixture of at least two most preferred.

Since TFE is considerably more reactive in the polymerization than HFP,an excess of HFP is needed to achieve the desired polymer composition.Typically this also means that at the end of the polymerization, much orall of the TFE will have polymerized, but there will be a considerableamount of unpolymerized HFP. Typically the TFE will be about 1 to 15mole percent of the total amount of monomer being fed to the process,with the HFP and other monomer(s) (if present) being the remainder. Theaverage residence time is the average amount of time any of the materialfed to the reactor actually spends in the reactor, and is a function ofthe volume of the reactor and the volumetric flow of the processingredients through the reactor. A preferred residence time is about 20sec to about 10 min, more preferably about 40 sec to about 2 min. Whenthe process fluids are being added to the reactor, it is preferred ifthey are preheated just before they enter the reaction to a temperaturesomewhat less than that of the actual reactor temperature, about 20° C.to about 100° C. less. This allows one to maintain a uniform constanttemperature in the reactor itself, and for the newly added materials tostart the polymerization reaction immediately upon entry to the reactor.

The polymer suitable for the practice of the present invention, as soproduced, comprises at least about 10 mole percent, preferably about 30to about 50 mole percent, of monomer units derived fromhexafluoropropylene, —CF₂—CF(CF₃)—. The polymer further comprises 0-50mole percent, preferably 25-50 mole percent, of monomer units derivedfrom tetrafluoroethylene, —CF₂—CF₂—. The polymer further comprises about10-75 mole percent, preferably 10-25 mole percent of one or more monomerunits (III) selected from the group consisting of:

—CF₂CFX—, —CH₂—CHR—, —CH₂CF₂—

wherein X is H or perfluoroalkoxy having 1-20 carbons, and R is H,alkyl, alkoxy, perfluoroalkyl, or F. Preferably (III) is a repeat unitderived from vinyl fluoride, vinylidene fluoride, alkyl, includingfluoroalkyl, vinyl ethers, (perfluorobutyl) ethylene, or ethylene. Mostpreferably, a combination of at least two of these is employed.

One of skill in the art will appreciate that many compositions areencompassed within the range suitable for the practice of the presentinvention, and that, particularly at concentrations of HFP below 30mol-%, not all such compositions will exhibit sufficient amorphouscharacter to be suitable for the practice of the present invention,while others may have little practical value. However, since the repeatunits (III) tend to disrupt crystallinity and reduce glass transitiontemperature while the repeat units derived from TFE (II) tend toincrease crystallinity and increase glass transition temperature, theeffects of the two groups of repeat units can be traded off against oneanother. Since some of the repeat units (III), such as vinylidenefluoride, vinyl fluoride, and ethylene, themselves form crystals at highconcentrations, while others, such as perfluoroalkyl vinyl ethers orperfluorobutyl ethylene, tend to limit molecular weight, it is found tobe particularly effective to employ two or more repeat units (III) incombination, for a total concentration of 10-25 mol-%.

For the purposes of the present invention, the polymer is preferably alow-viscosity liquid at the temperature of the stone surface to which itis applied in order to enhance the uniformity of coating and achievegood coating distribution in a matter of minutes to hours. The desireddegree of liquidity is achieved when the glass transition temperature ofthe substantially amorphous polymer of the invention is below thetemperature of application. Additionally, the viscosity of the liquidpolymer is determined in part by molecular weight, with lower molecularweight generally associated with lower viscosity. Vinyl fluoride,methyl-vinyl ether, ethylene and similar comonomers each tend to limitthe molecular weight of the finished polymer. Thus, one way to tune thepolymer viscosity to achieve the optimum for a particular application,is to manipulate the concentrations of those monomers in the polymer. Onthe other hand, when a solvent is employed during the application of thecoating, low molecular weight and low glass transition temperatureenhance the solubility of the polymer in the chosen solvent.

In a preferred embodiment, the polymer suitable for the practice of theinvention is a liquid, and may be applied to a stone surface directly,without dilution. However, it is preferred to first dissolve the polymerin a solvent to achieve the optimum control over uniformity andthickness of coating.

Coatings formed from the polymer of the invention are particularlyuseful because of the inherent properties of the polymer, such as lackof crystallinity (polymer is transparent), low surface energy (and hencepoor wetting by water or most organic liquids) while exhibiting highsurface coverage of stone, low dielectric constant, low index ofrefraction, low coefficient of friction, low adhesion to othermaterials, etc.

In the practice of the present invention, one or more of thesubstantially amorphous fluoropolymers hereinabove described is appliedby any convenient method to the surface of the stone which is to beprotected from the effects of water and environmental pollutants. It isimportant that the coating provide a barrier to liquid water withminimal effect on the natural water vapor permeability of the stone. Oneway of achieving this is to provide a durable coating in as thin a layeras possible on the wall surface of each pore of the stone withoutactually filling or blocking the pore. This is achieved by using amaterial of the lowest possible surface tension. Coating materials whichexhibit a desirable combination of properties are characterized bypendant groups comprising perfluorinated functional groups in sufficientconcentration that the surface presented to incident liquid water suchas rainfall is characterized by a high density of the perfluorinatedgroups and a consequently very low surface tension. In the mostpreferred embodiment, the polymer suitable for the practice of thepresent invention comprises 30-50 mol-% of repeat units derived fromHFP. The resulting low surface tension is the source of thethermodynamic driving force for complete wetting of the pores in stoneas well as for the liquid water repellency of the coated stone. Toreduce the kinetic barrier to complete pore wetting, the viscosityshould be as low as possible. This represents a particularly desirableattribute of the method of the present invention because thesubstantially amorphous fluoropolymer employed in the method of thepresent invention readily forms low viscosity solutions in a number ofconvenient solvents.

While in no way limiting the scope of the invention, it is estimatedthat the viscosity of the coating during application of the coating tothe stone is preferably less than about 10 Pa-s to achieve optimumcoating performance. It will be obvious to one of skill in the art thatwhile it is desirable to employ materials which afford low viscositysolutions, usually associated with low molecular weight or non-polymericmaterials, the materials so employed cannot be of such low molecularweight that they evaporate from the stone surface.

It is further preferred that polar groups should be present in thecoating material to promote adhesion of the coating material to thestone surface and decrease the tendency of the coating material tocontinually penetrate to the interior of the stone and reducing surfaceefficacy in terms of liquid water repellency. Esters, amides, —CH₂CF₂—and adjacent —CHCF— moieties are examples of such adhesion-promotingpolar groups.

According to the method of the present invention, the substantiallyamorphous fluoropolymer can be dissolved in a solvent which acts as avolatile diluent in the spraying operation to afford fast penetration atthe early stages of coating while providing a high degree of controlover the viscosity, the uniformity of coating and the coating thickness.

Solvents suitable for the practice of the present invention includeacetone, methyl-ethyl ketone, ethyl acetate, t-butyl acetate,hydrochlorofluorocarbons, perfluorocarbons. In the most preferredembodiment, the substantially amorphous fluoropolymer of the presentinvention is dissolved in supercritical CO₂ according to the methodsdescribed in Carbonell et al., WO 99/19080 or in the alternative in U.S.Pat. Nos. 4,923,720; 5,108,799; 5,290,603; and 5,290,604. To achievesolubility in relatively low pressure carbon dioxide, which is desirablefor on-site application of coatings, the combination of the molar ratioof hydrogen to fluorine attached to the polymer backbone, the “H:Fratio”, is about 0.3 to about 1.0 as described in U.S. Pat. No.6,034,170. If the polymer molecular weight is less than about 5000 Da, asomewhat broader H:F ratio can be sustained.

Spray-coating of stone is preferably effected from CO₂ solutions of 75weight % or less polymer at 40 to 70° C., 2000 to 4000 psi. To promotepolymer absorption into the stone it might also be preferable to add upto about 40 weight % acetone, t-butyl acetate, Oxol 100(4-chlorobenzotrifluoride), or other such environmentally friendlydiluents to the substantially amorphous fluoropolymer.

It will be understood by one of skill in the art that numerous chemicalcompounds have been identified which may serve as the supercriticalfluid for the substantially amorphous fluoropolymer coating compositionof the invention. However, CO₂ is by far the preferred compound becauseof the low cost, low toxicity, ready formation of a supercritical fluid,and low environmental impact.

The substantially amorphous fluoropolymer component of the coatingcomposition is generally present in amounts ranging from 1 to 80 weightpercent, based upon the total weight of the coating composition.Preferably, the substantially amorphous fluoropolymer component shouldbe present in amounts ranging from about 15 to about 70 weight percenton the same basis.

The supercritical fluid diluent should be present in such amounts that aliquid mixture is formed that possesses such a viscosity that it may beapplied as a liquid spray. Generally, this requires the mixture to havea viscosity of less than about 300 centipoise at spray temperature.Preferably, the viscosity of the mixture of components ranges from about5 centipoise to about 150 centipoise. Most preferably, the viscosity ofthe mixture of components ranges from about 10 centipoise to about 50centipoise.

The supercritical carbon dioxide fluid is most preferably present inamounts ranging from about 30 to about 85 weight percent on the totalcompositional weight, thereby producing a mixture having viscositiesfrom about 10 centipoise to about 50 centipoise at spray temperature.

It is not necessary to form a preliminary solution or dispersion of thepreferred substantially amorphous fluoropolymer composition in order toform a low-viscosity solution or dispersion suitable for mixing with theCO₂. It is however optional to add a third component to the coatingcomposition of the invention, the third component comprising one or moreorganic solvents employed for the purpose of improving viscosity controlduring spraying and “laydown” of the coating material on the stone.

The organic solvents suitable for the practice of the most preferredembodiment of the invention generally include any solvent or mixture ofsolvents that is miscible with CO₂, is a good solvent for thesubstantially amorphous fluoropolymer, and is fugitive at thetemperature at which the coating is being applied to the stone, normallyat temperatures of about 0° C. or above. Preferably, the solvent is alsoenvironmentally friendly. Suitable organic solvents include acetone,methyl-ethyl ketone, ethyl acetate, t-butyl acetate,hydrochlorofluorocarbons, and perfluorocarbons with acetone,methyl-ethyl ketone, ethyl acetate and t-butyl acetate preferred.

The coating composition of the invention is sprayed onto a substrate toform a liquid coating thereon by passing the liquid mixture underpressure through an orifice into the environment of the substrate toform a liquid spray.

Spray orifices, spray tips, spray nozzles, and spray guns used forconventional airless and air-assisted airless spraying of coatingformulations such as paints, lacquers, enamels, and varnishes, aresuitable for spraying the coating composition of the present invention.The spray pressure used in the practice of the present invention is afunction of the specific coating formulation. In the case ofsupercritical fluid solutions, the minimum spray pressure is at orslightly below the critical pressure of the supercritical fluid.Generally the pressure will be below 5000 psi. Preferably, the spraypressure is above the critical pressure of the supercritical fluid andbelow 3000 psi. If the supercritical fluid is supercritical carbondioxide fluid, the preferred spray pressure is between 1070 psi and 3000psi. The most preferred spray pressure is between 1200 psi and 2500 psi.

The spray temperature used in the practice of the present invention is afunction of the coating formulation. The minimum spray temperature isabout 31° C. The maximum temperature is determined by the thermalstability of the components in the liquid mixture. The preferred spraytemperature is between 35° C. and 90° C. The most preferred temperatureis between 45° C. and 75° C. Generally liquid mixtures with greateramounts of supercritical carbon dioxide fluid require higher spraytemperatures.

One of skill in the art will recognize that the method of the presentprocess, while specifically directed to the protection of stone, mayequally be employed to apply coatings to a variety of substrates.Examples of suitable substrates include but are not limited to metal,wood, glass, plastic, paper, cloth, ceramic, masonry, stone, cement,asphalt, rubber, and composite materials.

Through the practice of the present invention, coatings may be appliedto substrates in thicknesses of from about 0.5 to 100 micrometers.Preferably, the coatings have thicknesses of from about 1.0 to about 15micrometers, while most preferably, their thicknesses range from about1.5 to about 10 micrometers.

The method of the present invention provides a considerable benefit inthat the substantially amorphous fluoropolymer coating may be readilyremoved using common solvents such as acetone, methyl-ethyl ketone,ethyl acetate or t-butyl acetate, if it should be deemed desirable atsome point in time following the application thereof.

The coatings on stone produced by the practice of the present inventionare highly beneficial to the purpose of protecting the stone fromenvironmental degradation. Two key attributes which are indicative ofsusceptibility to weathering are water absorption, typically bycapillary action through the porous stone structure, and water vaporpermeation rate. It is highly desirable that the water absorption ofnormally highly absorbent stone be reduced by as large a factor aspossible, while water vapor permeability, normally high as well, bemaintained at a high level. The coated stone of the present inventionprovides high levels of water vapor permeability by virtue of the thincoatings which are found to be effective in providing the desired highresistance to water penetration.

The method of the present invention and the properties of the coatedstone compositions provided thereby are further illustrated in thefollowing specific embodiments.

EXAMPLES

In the following examples, a pressure cell as described in Tuminello etal., J. Appl. Polym. Sci., 56, 495 (1995), was used to evaluate thesolubility of the substantially amorphous fluoropolymer specimens belowin CO₂. The total volume of the cell was about 3.0 ml. Solid fluorinatedmaterial solute sufficient to make about a 17 volume percent solutionwas added to the cell first. A vacuum was applied for a few minutes andthen liquid CO₂ was added until the cell was filled at its vaporpressure, about 6.2 MPa (900 psi). Pressures could be increased to ashigh as 31.7 MPa (4600 psi) by pushing a piston through a manifoldloaded with CO₂. Temperature was increased to as high as 100° C. with anelectrical heating band around the pressure chamber. Temperatures as lowas about −10° C. were achieved by removing the heating band and packingdry ice around the cell. Cloud points were determined by visualobservation through the sapphire windows provided on the cell. The cloudpoint was determined at constant temperature with decreasing pressureand is defined as that pressure at which the mixture became so opaquethat it was no longer possible to see the stirring paddle inside thecell. Cloud point data for each sample are listed below.

In the following examples, the procedures followed in determining waterabsorption and permeability were essentially those described in Italianstandard test methods AA. VV, Assorbimento di acqua per capillarità,Raccomandazione NORMAL 11/85, CNR-ICR, Roma 1985 and 7 AA. VV,Permeabilità al vapor d'acqua, Raccomandazione NORMAL 121/85, CNR-ICR,Roma 1985.

Two stone substrates were employed, each in the form of prism-shapedspecimens 5×5×2 cm in size. They were:

(a) Marble—White Carrara marble with grey veins, 99% calcite, polygonalstructure and fine grains. Total porosity=3.83±0.2%; saturationindex=7.4±0.6%.

(b) Biocalcarenite—Lecce stone composed of Foraminifera with calcareousshell, glauconite grains and very small fragments of quartz. The clastsare bound by a micritic calcitic cement with a low clay content. Totalporosity=32 to 40%; saturation index=65±5.0%.

In each example, the average of the results obtained on three separateprism shaped specimens was determined. Five untreated stone specimens ofeach type were retained as controls. The stone specimens were maintainedin a dessicator containing CaCl₂ until a constant mass was reached usinga lab balance of precision of ±1 mg.

The coating was applied to one face of each stone specimen by paintingwith a brush as uniformly as possible. This was done after removing thestone from the dessicator. Coating thickness was determined by weighingbefore and after treatment. The painted stone specimens were then leftat room temperature in ambient air for one week to evaporate the solventand then placed in a dessicator along with the control specimenscontaining CaCl₂ until constant mass was achieved.

Each stone specimen thus brought to constant mass, was removed in turnfrom the dessicator and placed in contact with a stack of filter paper(1 cm thick; 9 cm diameter) soaked in distilled water. The amount ofwater absorbed by capillarity was determined by weighing the sampleafter a fixed time (marble—60 min.; biocalcarenite—20 min.) Protectiveefficacy (EP %) was calculated by the following expression:${E_{P}\quad \%} = {\frac{\left( {A_{UN} - A_{T}} \right)}{A_{UN}} \cdot 100}$

where A_(UN) and A_(T) are the amounts of water absorbed by theuntreated and treated samples, respectively.

In the ideal there would be no water absorption, or E_(p)%=100. In thecurrent state of the art, a very good level of efficacy is considered tobe 80 to 90%.

Each stone test specimen was mounted as a lid to a poly(vinyl chloride)test cell containing 10 ml of distilled water. The cell was equippedwith neoprene gaskets to keep the sample in place while leaving an areaof about 16 cm² through which water vapor could permeate. The cell wasthen placed in a thermostatic drybox maintained at a constanttemperature of 25.0±0.5° C., and containing a sufficient amount ofsilica gel and calcium chloride to maintain constant relative humidityof 2 to 5%.

A balance was placed in the drybox to monitor weight changes in the cellwithout the need to open the drybox. The weight of each cell wasmonitored every 24 hours for several days. Weight loss became constantafter a few days. The permeability (P) of the surface of the stone towater vapor was calculated using:

P=M/A (g/m ² in 24 hrs.)

where M is the amount of water, in grams, lost in 24 hours and A is theevaporating area, in m², of the system.

The reduction in permeability (R_(p)%) due to the treatment is definedas:${R_{P}\quad \%} = {\frac{\left( {P_{UN} - P_{T}} \right)}{P_{UN}} \cdot 100}$

where P_(UN) and P_(T) are the permeability of the untreated and treatedsamples, respectively. This procedure is described in more detailelsewhere. The best performance is to have permeability matching that ofthe untreated sample, or R_(p)%=0.

Examples 1 and 2

Following the practice of Example 1 of U.S. Pat. No. 5,478,905, themonomers along with a trace of nitrogen trifluoride were compressed to103 MPa and bled through a tubular reactor maintained at 300° C. and96.5 MPa. After a ˜1 minute residence time, a solution of polymer insupercritical monomer phase was withdrawn from the back end of thereactor. The solution thus withdrawn was reduced to atmospheric pressureand the polymeric residue collected and devolatilized.

A 25 ml loop off the feed line to a 3.8 liter stirred autoclave wasfilled with 440 psig of nitrogen trifluoride. The 3.8 liter autoclavewas then filled via the feed line with 60 g of tetrafluoroethylene, 2000g of hexafluoropropylene, 20 grams of vinyl fluoride, and 20 g ofethylene, using a portion of the hexafluoropropylene to blow thenitrogen trifluoride into the autoclave. The liquid monomer phase waspumped off the bottom of the autoclave, pressurized to ˜103 MPa, andthen recirculated back to the autoclave. After at least 10 minutes ofsuch recirculation, monomer was bled off the recirculation loop at ˜10to 12 grams/minute though a 225° C. preheated line to a 10 cc reactormaintained at ˜96.5 MPa and 300° C., followed by collection atatmospheric pressure. Flow rate through the reactor was ca. 10-12 g/min.Over a period of 120 minutes about 1300 g of monomer were passed throughthe reactor. Letting the reaction mixture back down to atmosphericpressure gave a yellow, foamy fluid that was allowed to first evaporatedown overnight and then dried further overnight in a 150° C. vacuumoven. This gave 176 g of a highly viscous fluid having an inherentviscosity of 0.067 in CF₃CFHCFHCF₂CF₃ solvent at 25° C. The compositionwas found by NMR to be 12.7 mole % vinyl fluoride, 38.6 mole %hexafluoropropylene, 23.0 mole % ethylene, 25.7 mole %tetrafluoroethylene. The glass transition temperature was −10° C. asdetermined by differential scanning calorimetry in the second heating at10° C./min heating rate in nitrogen.

CO₂ solubility was determined according to the method hereinabovedescribed, and the results are shown in Table 1.

TABLE 1 CO₂ Solubility Temperature (° C.) Cloud Point (psi) 21 1810 242180 33 2550 34 2800 37 2810 42 3000 45 3220 47 3290 49 3710 51 3580 523650

1 g of the solid polymer prepared above was dissolved in 99 g of1,1,2-trichlorotrifluoroethane at room temperature. The resultingsolution was applied to three stone specimens each of the white Cararramarble (Example 1) and Lecce stone (Example 2), as hereinabovedescribed. The specimens were allowed to stand for 1 week, after whichthey were subject to the procedures of dessication, water absorptiondetermination, and water vapor permeability according the methodshereinabove described. Results are shown in Table 2.

Comparative Example 1 CF₃—[CF(CF₃)CF₂O]m—(CF₂O)n—CF₃

The test procedures of Example 1 were followed employing Fomblin® YR, aperfluorinated polyether available from Ausimont/Montefluos,Montedison/Montefluos Group, Milano, Italy. Fomblin® YR is the materialcurrently preferred in commercial stone preservation applications. Stonetest specimens were prepared and tested as hereinabove described. Onlythe biocalcarenite was tested. The amount of material applied was thatfollowed in current commercial practice. Results are in Table 2.

TABLE 2 Results of Coating on Stone Protective Reduction in CoverageEfficacy Permeability Example Substrate (g/sq.m.) (E_(P) %)* (R_(P) %)**1 Marble  8.5 ± 0.40 84 ± 3 38 2 Biocal- 18.8 ± 0.78 85 ± 3 30 careniteComp. Ex. 1 Biocal- 49 18 Not Determined carenite Control Marble None  0 0 Control Biocalcare- None  0  0 nite *Goal is 100% **Goal is 0%

Comparative Example 2

Following the method described in F. Piacenti and M. Camaiti, J.Fluorine Chem., 69 (1994), 227-235, the monofunctional acid fluorideprecursor of a random perfluoropolyether of similar structure to the onein Comparative Example 1 was esterified and then condensed withhexamethylene diamine to form the diamide functionalizedperfluoropolyether material with a MW of about 1800 Da. This material isconsidered the state of the art for providing a combination of highwater repellency and low water permeability, as described in F.Piacenti, “The Conservation of Monumental Buildings: Recent ScientificDevelopments”, a lecture presented at the 2nd International Congress onScience and Technology for the Safeguard of Cultural Heritage in theMediterranean Basin—Paris—Jul. 5 to 9, 1999.

Biocalcarenite specimens were coated with 48 g/m² of theperfluoropolyether diamide so prepared according to the methods ofComparative Example 1. E_(p) was 55% as determined as hereinabovedescribed.

What is claimed is:
 1. A process for protecting stone comprising:contacting stone with a substantially amorphous fluoropolymer comprising(a) 30 to 50 mole percent of repeat units of the formula—CF₂—CF(CF₃)—  (I)  derived from hexafluoropropylene; (b) 25 to 50 molepercent of repeat units of the formula —CF₂—CF₂—  (II)  derived fromtetrafluoroethylene; and (c) about 10 to about 25 mol- % of repeat unitsselected from the group consisting of —CF₂CFX—, —CH₂—CHR—,—CH₂CF₂—  (III) and mixtures thereof wherein X is H or perfluoroalkoxyhaving 1-20 carbons, and R is H, alkyl, alkoxy, perfluoroalkyl, F. 2.The process of claim 1 wherein the repeat unit (III) is selected fromthe group consisting of vinyl fluoride, vinylidene fluoride,perfluoroalkylvinyl ethers, perfluorobutylethylene, alkylvinyl ethers,ethylene, and combinations thereof.
 3. The process of claim 1 whereinthe amorphous polymer is a liquid.
 4. The process of claim 1 wherein thebackbone H:F ratio of said amorphous fluoropolymer is about 0.3 to about1.0.
 5. The process of claim 1 further comprising forming a solution ofsaid substantially amorphous fluoropolymer prior to contacting saidstone therewith, wherein said stone is contacted with said solution ofsaid substantially amorphous fluoropolymer.
 6. The process of claim 5wherein said solution comprises supercritical CO₂.
 7. The process ofclaim 6 wherein said solution further comprises an organic solvent whichis fugitive at a temperature at or above about 0° C.
 8. The process ofclaim 7 wherein said organic solvent is selected from the groupconsisting of acetone, methylethyl ketone, ethyl acetate, t-butylacetate, hydrochlorofluorocarbons, and perfluorocarbons.
 9. Acomposition comprising stone and a substantially amorphous fluoropolymerin the form of a coating thereupon, the substantially amorphousfluoropolymer comprising: (a) 30 to 50 mole percent of repeat units ofthe formula —CF₂—CF(CF₃)—  (I)  derived from hexafluoropropylene; (b) 25to 50 mole percent of repeat units of the formula —CF₂—CF₂—  (II) derived from tetrafluoroethylene; and (c) about 10 to about 25 mol- %of repeat units selected from the group consisting of —CF₂CFX—,—CH₂—CHR—, —CH₂CF₂—  (III) and mixtures thereof wherein X is H orperfluoroalkoxy having 1-20 carbons, and R is H, alkyl, alkoxy,perfluoroalkyl, F.
 10. The composition of claim 9 wherein the repeatunit (III) is selected from the group consisting of vinyl fluoride,vinylidene fluoride, perfluoroalkylvinyl ethers, perfluorobutylethylene,ethylene, alkylvinyl ethers and combinations thereof.
 11. Thecomposition of claim 9 where said coating has a thickness in the rangeof 1.5 to 10 micrometers.
 12. The composition of claim 9 wherein thebackbone H:F ratio of said amorphous fluoropolymer is about 0.3 to about1.0.